CA1290134C - Infiltration processing of boron carbide-, boron-, and boride- reactive metal cermets - Google Patents

Abstract

INFILTRATION PROCESSING OF BORON CARBIDE-, BORON-, AND BORIDE- REACTIVE METAL CERMETS

ABSTRACT Of THE INVENTION

A chemical pretreatment method is used to produce boron carbide-, boron-, and boride-reactive metal composites by an infiltration process. The boron carbide or other starting constituents, in powder form, are immersed in various alcohols, or other chemical agents, to change the surface chemistry of the starting constituents.
The chemically treated starting constituents are consoli-dated into a porous ceramic precursor which is then infil-trated by molten aluminum or other metal by heating to wetting conditions. Chemical treatment of the starting constituents allows infiltration to full density. The infiltrated precursor is further heat treated to produce a tailorable microstructure. The process at low cost pro-duces composites with improved characteristics, including increased toughness, strength.

Description

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BACKGROUND OF THE INVENTION

The United St~ates Government~has rights ln~this~

nventlon pursuant to Contract No. w-7405-ENG-48 b~etween the United States Department of Energy and the Universlty of California for the operation of Lawrence Livermore National Laboratory. ~ ~
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The present invention relates to processes for making~metal-ceramlc c~mposites, and more particularly to infiltration methods.

United States Patent 4,605,440 is directed to boron carbide-reactive metal composites, particularly B4C-Al, and methods for making same. Fully dense composites with tailorable microstructures can be pro~uced. However, Lt is desirable to find alternate methods for producing these composites~
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It is also desirable to form composites of reactive metals ~nd boron or borides.
The concep~ of infiltrating a molten metal into a ceramic sponge is known and has been c~rried out by various different methods. U. S. Patent No. 3,864,154 by Gazza et al shows a method of infiltrating metal into a porous cera~ic compact without any treatment of the compact. A compact of silicon boride, aluminum boride or bDron is positioned between powdered aluminum in a vacuum:
IO furn2ce. Full density is not achieved. To infiltrate Al nto B4C, Si must be added to the Al as a wetting agent;~
~; Al alone could not be infiltrated~into B4C. U. S.
Patent No. 3,725,015 by weaver shuws:an infi~ltration ;~
~: methoa in which a precursor is formed:wi~h a carbon ~ :
:~ 15 containing substance and the precursor is heated to : ~ :
decompose the~car~bon ccntainin~substance to form a car~bon residue. U. S. Patent No. 3,718,441 by Landingham shows ~:
n infiitraticn methcd in which the -etal is treated by~ ~ ;
heating at low pressure to remove an oxide fi~lm and ~acil~tate wetting. Other related art includes U. S.
Patent No. 2,612,443, Goetzel et al, WhlCh shows an ~ infiltrat~on method in WhiCh a skeleton ~ody ~s formed in : a mold:by heat trea~ing/sin~ering and molten metal is forced into the pcres by fluid pressure; U. S. Patent No.
25~ Z,581,252, Goetzel et~ al~ and~U. S. Patent Nc. ~ ~ SS~ :

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Ellis et al, which are directed to infiltrating a skeleton in a mold by applying pressure to the infiltrant; and U. 5. Patent No. 2,672~426, Grubel, which shows a ~ethod of making and sintering a metal compact and impregnating with a molten ceramic. None of these patents show the infiltration of aluminum into a porous boron carbide (B4C) sponge; Gazza shows that silicon must be added.
Previous attempts to infiltrate aluminum ~nto a B4C
sponge were unsuccessful since these methods were not ; 10 based on the chemical reaction kinetics and the mechanisms of liquid rearrangement whic~h are the basis of the pt sent ,~ :
~; invention. Recently Pyzik and Aksay have shown that, by:
thermal modification of the as-received B4C starting ~:
: ~` constituent, it is possible to infiltrate aluminum and ~: 15 aluminum alloys into a porous B4C sponge. The thermil odification process requires heating to high ; :temperatures, about 2000C~ :in a controlle-d atmosphere, : : thereby sintering the ceramic grains.
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The process of the present invention is superior ~
to the pr~or art for the following reasons: -: l. It 1s more economical than previously descr~bed i nf i 1 trat i on processes .

2. It results in a microstructure that offers ~mproved propertles over other processes~
~5 3. It is possible to acoieve tailorable propertie~ in a fully dense compos~ite ~ody.

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Accordingly, it is an object of the invention to provide an infiltration method for producing 5 B4C-reactive metal, and particularly B4C-Al, composites.
It is another object to provide fully dense composites by an infiltration process.
It is also an object to provide a method of producing ~oron- and boride- reactive metal composites by infiltration. ~ .

SUMMARY_OF THE INVENTION
The invention is a process for the infiltration of molten reactive metals into cbem1cally pre-treated boron carbide, boron, or boride starting constituents :.
(powders, fibers, etc.) that have been consolidated into a ceramic precursor or sponge via conventional or -colloidal-chemical-casting techni~ues, or by injectlon molding processes. The process includes the steps of chemical pretreatment of the starting constituents, consolidation of the chemically pretreated starting : constituents into ~ precursor, and inf~ltration of the reactive metal into the precursor, The infiltrated precursor can then be further heat treated to prcduce desired reaction products 1n a tailored microstructure~

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- 4a - 61051-2091 Thus the present invention provides a method of fabricating metal-ceramic composites from previously formed ceramic precursor starting constituents selected from boron-carbide, boron and borides and metals reactive therewith selec-ted from reactive metals, alloys thereof, and compounds thereof which reduce to reactive metals or alloys thereof, comprising:
chemically pretreating the previously formed starting constituents of a ceramic precursor; consolidating the chemically pretreated starting constituents into a porous ceramic precursor; infiltrat-ing molten reactive metal into the chemically pretreated ceramic : precursor; wherein the step of chemically pretreating the starting constituents of the ceramic precursor alters the surface chemistry to enhance infiltration of the precursor by the mo.lten reactive metal by slowing the kinetics of reactiOn relative to the kinetics of densificatlon. ~ ~

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, ~2~ L34 ThiS invention is superior to the prior ~rt in that it is more economical than conventional processing methoas, such as hot-isostatic pressing and hot pressing. ~;
The process also costs considerably less than the recently developed method of infiltration using thermally-mo~ified precursors.
The invention not only offers cost savings but results in a product with increasea toughness, strength, -~
and thermal and electrical conductivity in these materiats - 10 over the same processed by other methods.
~ ~ e~ q'~O .J/~ r~g The key to this process is ~14h~ q~ ~he co~ f ~ e n fs surface chemistry of the starting ~?~btWYt~ hen the mechanisms of infiltration are inhibited by undesirable~
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phases (such as oxides and/or stoichiometric deficlencies) ; 15 at the sur~ace of these starting constituents, it is possible to chemically convert or chemically convert and then subsequently pyrolyze them at low temperatures into phases which are conducive to the prooess. By Chemically controtling the surface of the starting constituents, full ~0 density, ~.e., greater than 99X of theoretical density, can be achieved by infiltration.
Addit~onal advantages of the new process include the ability to fabricate yradient microstructures with pr~perties that can be tailored to meet use demaods which vary along the finished part. The invention could also be :
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' incorporated with injection molding in a two-step process. Here, small highly-configured geometries can be molded using binders containing the appropriate chemical pre-treatment agents as the first step. Then ~he binder is volatilized from the precursor making it ready for subsequent infiltration in the second step.
This new process is particularly applicable to the fabrication of boron carbide-aluminum cermets.
However, the process can also be used with boron and boride starting constituents, and other reactive metals, or alloys~ or compounds which reduce to the metal or alloy during the process.

BRIEF DESCRIPTION OF THE DRAwl~lGS
Figures lA,B show two B4C precursor tSponge) morphologies produced by the thermal modification process;
Figures 2A,~ show two infiltrated microstructur2s of titanium carbide (TiC~ cermet having continuous and discontinuous carbide networks, respectiYely;
Figures 3A,B,C are schematic diagrams of the infiltration of a metal lnto a precursor; and Figure 4 shows the infiltrant-precursor interface in an Al-B4C compDsite.

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DETAILED DESBRIPTION OF THE P~EFERRED EMBODIMEN~
The infiltration process o~ the present lnvention is a chemical modi~ication method. The process ~s more economical than thermal modification methods because chemical treatments can be done at room temperatures with relatively inexpensive chemicals, whereas thermal treatments require costly furnace equipment to obtain temperatures between 1800C and 2250C in controlled atmospheres (i.e., with minimal oxygen contamination, etc).
~ The chemical modification process of the IO invention results in microstructures that haYe improved propertîes over those produced by thermal ~odif kation processes. Tne microstructure characteristics of concern can be illustrated by examining FigsO lA,B and 2A,B; these Figures do not show composites made by the process of the invention but serve to illustrate important features.
Figures lA,B show two different B4C sponge ~orphologies, both obtained by ther~al processes. The 27 volX porosity sponge was achieved by heating previously cold-pressed B4C powders to 2100C for 30 Inin~ in an argon atmosphere. The 39 vol% porosity sponge was fabricalted in a similar manner but at 1900C for 30 minO The key d~fference in the sponge morphologies of Flgures lA,B is t~e degree of interconnecting of the 84C grains. The 2100C microstructure has been partially sintered to a ..

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~L29~ 4 - B -greater extent than the 1900C microstructure. Both of these microstructures can be infiltrated w~th aluminum or aluminum alloys because thermal treatments result in a compositional modification of the B4C surface. Th~s S ~llows the kinetics of densification to proceed faster tnan the kinetics of interfacial chemiclal reactions resulting in dense ~inal bodies. Figures 2A,B show two infiltrated microstructures of prior art TiC cermets.
There is a greater degree of interconnected grains ~n Figure 2A over that of Figure 2B. The continuous ~nd the discontinuous carbide networ~s result in microstructLIres ,~
that have widely differing properties. The process of the present inYention includes control of the porosity and degree of interconnection. It is an object of the 1`5 invention to produce a disconnected morphology, i.e.~
wherein the ceramic grains are not fused together and are pre~erably surrounded by metal. The thermal modification `: :
process produces a connected morphology as a result of the high temperature sinter~ng. These property differences of composites prepared according to the invention compared to compusites prepared by thermal processes are qualitatively outlined in Table 1.

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TABLE I
MECHANICAL AND PHYSICAL PROPE~TY BEHA~IOR RELAr10NSllP
OF CERMETS TO CARBIDE SPONGE MORPHOLOGY
, Equal neight Carbide Morpholoqy Connected Disconnected (Thermal (Chemical Pro~rty Process) Process) : Hardness increases decreases Toug~ness decreases increases Brittleness increases decreases ~: 10 Ductility decreases increases odulus of Rupture decrea~es increases ~ ~.
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~ ~ ~ Elastic ~odulus increases decreases :, ' ., Impact R~esistance decreases increases -: Ther~al Conductivitydecreases increases~
:15~ Electrioal Conductivity decreases1ncreases Neutron Absorption unchan~ed uncha~nged The ~nfiltration step of the process is schematically shown in Figures 3A,B,C. The chemical modification process is performed on the B~C st~rting constituents prior. to preparation of the porous B4C

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precursor or sponge 10. Once the chemically modified precursor 10 is prepared, aluminum metal 12 is placed on the precursor 10. The metal and precursor are heated to achieve the wetting condition so that molten metal 14 flows or infiltrates into the pore space 16 of the precursor~ By filling the pore volume with metal, a fully dense composite 18 is produced.
One major advantage of the infiltration process of the invention is that it allows the fabrication of graded" or gradient microstructures. The term graded"
;~ means that the metal content ~n the cermet component varies along one or more directions so ~he physical and mechanical proper~ies of any given section can accommodate the imposed con~itions which coulP also vary in tbese lS directions.
~ The fabrication of graded components can be ; ~accomplisbed by controlled colloidal consolidation of the B4C precursor after chemic~al mod1f~cations have Deen made to the B4C starting constituents. Collo1dal consolidation ~s a method whereby ~ne B4C star~ing particulates are suspended tempDrar~ly in a llqu1d medium. Then by selecting an electrostat1c. sterit, or comb1ned electrostatic and steric ~electrosteric) means of controlling the sol, it is poss~ble to control the forces be~ween particulates (wh~ch may also be classifi~d into : .

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~ 61051-20~1 controlled size distributions) to cause them to remain dispersed or allow them to flocculate. This control .results in a desired sponge morphology upon consolidating the green body by gravitational settling filtration casting (pressure or pressureless), centrifugal casting, or injection molding technologies.
The colloidal consolidation approach combined with controlled polymodal size distributions of B4C
starting constituents allows the fabrication of B4C
precursors with controlled porosities from less than 10 vol% to over 80 vol~. These pores are subsequently infiltrated with liquid aluminum, aluminum ailoys or :
; compounds which are reduced to alumlnum or aluminum alloys :
: : during the process, thereby offering a wide range of properties in the "as-infiltrated" cermet. :
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A colloidal consolidatlon method is described :
in United States Patent 4,605,440.
Other, more conventional, means can also be used to fabricate the B4C precursor after chemical 20 ~ modification o~f the starting~const.ituents. These lnclude cold pressing, warm pressing, (e.g., injection molding :~
with a binder phase), plasma jet coating, combustion synthes.i.s, hot pressing, hot-isostatie pressing etc. By seleetively loading (either with different powder .s"~ ~
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~29~134 densities or with different pressures) the pressing dies, cavities, or molds it is also possible to obtain gradient B4C sponge morphologies, but with less control than with the colloidal consolidation method previously described.
with either me~hod, however, the gradient morphology will be ~aintained after infiltrationO
The graded precursor can also be achieved on a shaped product, even when the starting sponge morphology is of uniform density, i.e., not graded. ~ith a selected B4C starting size distri~ution9 proper control of the infiltration process, and/or non-isothermal heat treatments, selected areas of the shaped product can be pr ferentially altered. Another importa~nt effect that can be achieved by the infiltration process is the integral attachment of a metallic aluminum or aluminum alloy ~ surface to the B4C precursot. This metal-enriched : surf~ace film ls ~erely an extension of the aluminum or aluminum alloy phase of the infiltrated structure and it may or may not be saturated With phases typical to B4C-At or B4C^Al-alloy cermets depending on the processing conditions employed. Tnis concept can be extended to the idea of ~ttacning a bUlk metal phase to the infiltrated cermet thereby creating a gr~dient microstructure. This is especially observed along ~he edge of the precursor-infiltrant interface where a ' :

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dispersion and sepanat~on of the carbide grains ls preponaerant. This phenomenon is illustrated in Ftgure 4 for an Al in~iltrant and B4C preoursor. Th1s type of product could have completely different properties ~t the S surface or surfaces of the part compared to ~ts bulk properties.
It is also possible to coinfiltrate the B4C
precursor with different metals or other infiltratable ~aterials to even further enhance the multiproperty character of the products obtained.
The chemical modification of B4C starting constituents involves immersing of the starting constituents in selected chemical solutions. The expos~ure of tbe B4C surfaces to these chemical solutions results lS in a chemical reaction at the B4C-solution interface.
The reac~ion products that form at the B4C-solution interface have stoichiometrically difFerent boron to carbon ratios than that of ~he init~al B4C starting constituent. It is primarily this difference in B:C
ratios at the surface that inhibits the reaction kinet k s and/or promotes the densification kinetics during the infiltration process.
Boron carbide (B4C3 ~s typically prepared by a carbotnermic reduction of boric anhydride ~B2~3, also called boron oxide, boron triox~de, boric oxide, or boron .

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~290134 sesquioxide) with carbon by the follow~ng chem kal reactiun:
~2500C
2B~03 + 7C ~ B4C t 6CO
Other, more sophisticated, ~anufacturing processes have been developed which also result 1n the production of boron carbide. Boron carbide ~s discussed herein is not limited to the stoichiometry of B~C9 but can exist as a homogeneous range of boron ~nd oarbon ctoms witn carbon contents between less than l atomX and ~5 ~3 h ~ro~ e~ ~/
atom~. Other boron carbides outside of this I~M~eR~e~
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range have also been reported. Therefore, this ~nvention is appl~icable to all types of bDron carbide starting constitue~nts, amorpnous or crystailine, regar~dless of stoichiometries. This invention is also applicable to :
boron or other boride starting constituen~s.
The key to th~s invention is not the control of ~ ;
the bulk chemistry of the B4C starting constituents but rather the~control of their surface chemistry. These starting constituents are typically in the form of powders from as large as several millimeters to as small as tens of angstroms in size. The smaller the size the l~rger the surface area per unit volume o~ B4C, hence the grea~er effect surface properties play on processing behavior.

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. . ~ . ' ~90~34 Any B4C star~ing constituent, either through incomplete reactions during ~anufacture or by the mere contact with an atmosphere that con~a~ns oxygen, will have a surface layer of B203 or other boron oxide stoichiometry at its surface as ~ contaminant. Other contaminants Gr impurities may also be present in the surface as well as in the bulk (e.g~, Fe, Ti, Al9 Si, Ca, Mg, Ni, Cr, Mn, Cu, Ag, Be, etc.). The level of these other impurities depends on the manufacturing process and is usually on the order of a few parts per million up tD
as much as a few tenths of a weight percent in the bulk ~ :
and even less at $he surface. The levels of these other contaminints dt the surface are insignificant compared to~
:: the amount of boron oxides (herein referred to as just ~ f ,~ eY~ fp B203 for brc~icty)-In addition to the B203 at the surface of the B4C starting constituents~ the presence of moisture in t~e atmosphere will cause some or all of the B203 surface to convert to boric acid (BH303, also called boracic acid or orthoboric acid). Somet~mes the B203 can exist in a hydrated state (B203-H~O).
~hen B203 and/or BH303 ~con~am~nated~
B4C starting constituents are used to prepare the B4C
precursor prior to ~nfiltrat~on, the surface chemistry at tlle ~nfiltr~nt-precursor interf~ce will not be conducive ~ ~ ' : :

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-` 12gO134 to the infiltration process for two reasons: (1) a large concentration of B203 and/or BH303 ~t the surface will, upon heating past 450C, form ~ l~quid phase which fills the pores of the precursor and inhibits infiltration of the molten metal; or (2) a small concentration of B203 and/or BH303 at the su~face will, upon heating past 450C, form a liquid phase which acts as a flux causing an increase in the reaction rates of the chemical reactions at the B~C-Al interface. This later condition will cause the reaction kinetics to be accelerated faster than the kinetics of densification (infiltration) thereby Ulocking up~ the microstrueture with residual porosity before full density is achieved.
In either case the presence of an undesired B:C ratio at .
: 15 the surface of the precursor is established.
~ hen the precursor undergoes the preYiously described thermal modi~ication process two results occur: :
; ~ First, the B203 and/or 3H303 ~s volatilized at temperatures near 2000C in an argon atmosphere. Second, ~-the free boron present at the ~nterface will form boron-nitrogen phases and/or a change in B4C
stoichiometry. The nitrogen is made available via the argon gas WhiCh can typically contain up to or more than 100 ppm N2. The formation of BN ls thermodynamically ~25 favored over that of B4C at temperatures near 2000C
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~9~)~34 (-12 kcal/mole versus -6 kc~l/mole for BN ~nd B4C, respectively at 2000~ Because we are dealing with surface chemistry, these effects do not have to be large in extent and usually occur within the first 30-50A of the surface. The removal of B203 and/or BH303 plus the presence Ot BN and/or B4C stoichiometry changes at the surface alters the B:C ratio such that the surface chemistry of the infi~trant-precursor interface is conducive to the infiltrat~on process. That is, tne kinetics of reaction arP slowed down while the system is in a condition which ~s thermodynam~cally favorable for : : :
~ wetting to occur; or the kinetics of densification ; .
~ (infiltration) are faster than the kinetics of chemical ~ ~
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reactions at tne B4~-Al interfaces.
I~n this invention, the B:C ratio is adJusted chemically rather than ther~ally. This ~s accomplished by immersing or washing the B4C starting constituents in a chemical substance which reacts with the B203 and/or BH303 to form trimethyl borate (C3HgB03~ also cal~ed ~oric acid, tr~nethyl ester) andtor~any other boron-carbon-hydrogen-oxygen chem~cal complex that w~ll pyrolyze upon heating pr1Or to or during the Infiltration process to the required B:C ratio or ratios. The starting con~stituents are immersed for as long as necessary to change the sur~ace chemistry, typicalIy a few hours to a few days or longer.

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12~)134 .
^ 18 -Some of the preferred chemical subs~ances that react wit~ B203 ~nd/or BH303 to form C3HgB03 and/or any other boron-carbon-hydrogen-oxygen chem1cal oomplex include, but ~re not limi~ed t~ the following:
1. Methanol (CH40 or CH30H, also called methyl alcohol, carbinol, wood spirit, or wood alcohol).
. 2. Anhydrous alcohol (C2H60H to which has been added some substance or substances which renders it ent~rely unfit for consumption 3S a beverage; e.g., ~10 methanol~ camphor, aldehol, amylalcohol, gasoline, t~d~ en c~ /
~t~ isY~Y~r~ , terpineol, benzene, c~ster oil, acetone, : ~ nicotine, aniline dyes, ether, cadmium iodide, pyridine bases, sulfuric acid, kerosene~ diethyl phtholate, etc.).

3. n-Propyl alcohol (C3H80 or CH3CH2CH~OH, also called l-propanol or propylic alcohol).
~: 4. Isopropyl alcohol (C3H80 or ;~ ~CH3HOHCH3, also oalled 2-propanol, isopropanol, secondary propyl alcohol, dimethyl carb~nol, or petrohol~).
5. n-Butyl alcohol (C4HloO or CH3CH2CH2CH20H, also called l-butanol~ butyl ; alcohol, or propyl carb~nol).
6, sec-Butyl Alcohol (C4HloO or CH3CH2C~(OH)CH3, also called Z-~utanols butylene hydrate, 2~hydroxy butanep or methyl ethyl carbinol).

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1290~34 7~ ~ert-Butyl a k ohol (C4Hlo0 or (CH3)3COH, also called 2~methyl-2-propanol, or trimethyl c~rbinol).
. 8. Any azeotrope of the above.
9. Any dilution of the the above.
10. Any hlgher alcohol (CnHmO), where n ~, 4 and m ~ 10), or azeotrope or dilution thereof.
11. 61ycerol (C3H803 or : CH20HCH0HCH20H, also called 1, 2, 3-propanetriol, glycerln, or trihydroxypropane).
12. Any glycerol derivative (CnHmOp9 where n 3 3, m ~ 6, and p ~ 2), including glyceraldehyde )~ glyceric acid (C3H6O4 or ~: CH2OHCH(OH)COOH)~, glycerol formal (C4H8O3), and ~ 15 glycidol (C3H602).
: 13- Any methyl- ~bietate (C21H32O2), : ~ acetate (C3H6O2 or CH3COOCH3). acetoac (C3H8O3 or CH3COCH2COOCH33, acetylsalicylate ( lOH10O4)~ acryl~te (C4H62 or CH2 CHCOOCH3~ benzoate ~C8H8O2 or C6H:5C00CH3), benzoylsalicylate (ClsH1204). butyl ketone (C6H120 or CH3COC4Hg), butyrate ~G5H1002 or CH3(CH2~2COOCH3)~ c~rb~tol (C5Hl2o3 or . cH3ocH2cH2~cH2~H2oHj~ carbonate (C3H603 or C0(0CH3)2), ~elloso~ve ~C3HgO2 or ~:90~l34 HOCH2CH20CH3), cellosolve ~cet~te (C5H1~03 or CH3~CH2CH200CCH3), or methylcellulose, or methylal (C3H~02 or CH2(0CH3)2) 14. Boiling water~
15. Cold water.
16. Any combination of the above, in part or whole 17. Any binder, for the purpose of injection molding, containing any of the above, in part or whole.

~he inflltration process according to the invention:can be summarized as follows:

Precursor starting constituents, typically in powder form, are immersed in a chemical substance as previously described.; The surface chemistry is altered to facilitate wetting.

Step 2 Ths chemically treated starting constituents are then consol~dated or packed into a porous ~spongeU or compact by colloidal chem1cal means followed by a selected casting method as previously explained to form the consol~dated precursor. Alternatively, other consolidation methods can be used.
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Step 3 The metal phase is placed adjacent to the consolidated precursor and the assembly is placed in a vacuum or inert atmosphere furnace.
Step 4 The assembly is heated to conditions which promote wetting.

The assembly can also be further heated to conditions which promote microstructural enhancement and/or gradiation as previously described.
Step 6 : The final part is removed from the furnace ~ ' and machined to its final dimensions (if not already processed to its desired configuration).
The:infiltration process of the inventlon is . carried out in accordance with the wetting and reaction conditions described in United States Patent 4,605,440.
By controlling the reaction conditions, e.g., temperature ~ 20 and time, the relative :: : ~ :
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- ~29~34 proportion of various phases in the composite microstructure is controlled.

Example 1 Start wit~ 1 micron average diamete~ B4C
powders. Soak these starting constituents in methanol for 10 days with continuous stirring. Filter cast this slurry into a mold of desired configuration. Remo~e casted precursor (sponge) from mold and place an amount of lu~inum equivalent to the calculated pore volume of the precursor on the sponge. Heat in a vacuum furnace:to 500C and hold for one hour (to remove volatiies), then increase temperature to llR0C for a long enough time to all:ow for eomplete`infiltration of aluminum into the precursor, then cool under vacuum. Remove fired part.
Final machine if necessary.
~, Example 2 Start with a polymodal distribution of 0.2 micron and 0.8 micron d~ameter B4C powders. Soak these starting const~tuents in methanol w~th ~ 3 wt.X
polyethylene glycol binder ad~ition for 10 days w~th continuous st~rring~ Filter Cdst tnis slurry through a filtration-funnel system and catch all starting constituents on f~lter paper. Scrape these starting :

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129~34 ~ 23 - :

constituents into a eold pressing die and press to 10,000 psi. Remove precursor from die and place ~n a crucible containing a bed of powdered aluminum equivalent to or greater than the calculated pore volume of the precursor.
Heat assembly in a vacuum furnace to 500C and hold for one hour, then increase temperature to 1050C for a long enough time to allow for complete infiltration of aluminum into the sponge, then drop temperature to 800C for 24 hours to enhance the microstructure. then c801 under vacuum. Remove assembly and machine to final dimensions.

Example 3 Start with 10 micron averagè diameter B4C.
Soa~ these starting constituents in isopropanol~for 10 days with continuous stirring. Filter cast this slurry~
through a filtration-funnel system and catch all starting constituents on filter paper. Scrape off these startîng const~tuents and mix them with a thermoplastic injection molding resin containing methylcellulose~(binder).
Injection ~old the mix into 3 desired mold configuration.
Remove tne consolidated B4C-resin mix and place in a vaouum oven and Uburn out" the b~nder leav~ng only the B4C sponge. Place an amount of aluminum alloy (e.g., 7075-T6) equivalent to the ealeulated pore volume of the preeursor on top of the sponge. Heat the assembly in a ~: , ~ ' , : . :' ' ~l2~0~34 Yacuum furnace to 1100C for 2 hours (to c~use both infiltration and microstructural enh~ncement). Remove fire~ part and finisn marhine if necessary.
,, The invention has been described with respect to the treatment o~ B4C starting constituents for the formation of B4C-Al composites. However, boron and borideS e.g.~ AlB12~ AlB2, TiB2, starting ; constituents can similarly be treated prior to the ~ , formation of the porous precursor. Reactive metals, or ~;
alloys thereof. or compounds which reduce to the metal or alloy, can then be infiltrated by heating to the wetting condition, and the infiltrated precursor can be ~urther ~;
heat treated to promote reactions to tailor the microstructure of the composite.
Changes and modifications in the specif~cally described embodiments can be carried out wit~out departing from the scope of the invention w~ich is intended to be `
limited only by the scope of the appended clai~s.

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Claims (24)

1. A method of fabricating metal-ceramic composites from previously formed ceramic precursor starting constituents selected from boron-carbide, boron and borides and metals reactive there-with selected from reactive metals, alloys thereof, and compounds thereof which reduce to reactive metals or alloys thereof, comprising:
chemically pretreating the previously formed starting constituents of a ceramic precursor;
consolidating the chemically pretreated starting constituents into a porous ceramic precursor;
infiltrating molten reactive metal into the chemically pre-treated ceramic precursor;
wherein the step of chemically pretreating the starting con-stituents of the ceramic precursor alters the surface chemistry to enhance infiltration of the precursor by the molten reactive metal by slowing the kinetics of reaction relative to the kinetics of densification.

2. The method of claim 1 wherein the step of chemically pretreating the starting constituents comprises chemically alter-ing the surface chemistry of the starting constituents to elim-inate phases which inhibit infiltration.

3. The method of claim 2 wherein the step of chemically pretreating the starting constituents comprises chemically converting phases which inhibit infiltration to phases which promote infiltration.

4. The method of claim 3 further including pyrolyzing the chemically converted phases at low temperature.

5. The method of claim 1 further comprising forming the starting constituents into particles prior to chemically pretreat-ing.

6. The method of claim 5 further comprising forming the particles with a controlled polymodal size distribution.

7. The method of claim 1 wherein the step of consolidating the starting constituents is performed by colloidal consolida-tion.

8. The method of claim 6 wherein the step of consolidating the starting constituents is performed by collidal consolidation.

9. The method of claim 1 wherein the steps of pretreating and consolidating the starting constituents are performed by injection molding the starting constituents using a binder con-taining chemical pretreatment agents.

10. The method of claim 1 wherein the step of pretreatment is performed by immersing the starting constituents in a chemical solution to produce chemical reactions at the surface of the starting constituents.

11. The method of claim 10 for boron-carbide starting con-stituents comprising changing the boron to carbon ratio at the surface of the starting constituents.

12. The method of claim 1 comprising chemically removing B2O3, BH3O3 and B2O3 H2O from the surface of the starting constituents.

13. The method of claim 10 comprising immersing the starting constituents in any of the following:
a. methanol (CH4O or CH3OH);
b. anhydrous alcohol (C2H6OH);
c. n-propyl alcohol (C3H8O or CH3CH2CH2OH);
d. isopropyl alcohol (C3H8O or CH3HOHCH3);
e. n-butyl alcohol (C4H10O or CH3CH2CH2CH2OH);
f. sec-butyl alcohol (C4H10O or CH3CH2CH(OH)CH3);
g. tert-butyl alcohol C4H10O or (CH3)3COH);
h. any azeotrope of the above;
i. any dilution of the above;
j. any higher alcohol (CnHmO), where n?4 and m?10), or azeotrope or dilution thereof;
k. glycerol (C3H8O3 or CH20OHCHOHCH2OH);
l. any glycerol derivative (CnHmOp, where n?3, m?6, and p?2), including glyceraldehyde (C3H6O3), glyceric acid (C3H6O4 or CH2OHCH(OH)COOH), glycerol formal (C4H8O3), and glycidol (C3H6O2);
m. any methyl-abietate (C21H32O2), acetate (C3H6O2 or CH3COOCH3), aceoacetate (C3H8O3 or CH3COCH2COOCH3), acetylsalicylate (C10H10O4), acrylate (C4H6O2 or CH2=
CHCOOCH3), benzoylsalicylate (C15H12O4), butyl ketone (C6H12O or CH3COC4H9), butyrate (C5H10O2 or CH3(CH2)2 COOCH3), carbitol (C5H12O3 or CH3OCH 2CH2OCH2CH2OH), carbonate (C3H6O3 or CO(OCH3)2), cellosolve (C3H8 O2 or HOCH2CH2OCH3), cellosolve acetate (C5H10O3 or CH3 OCH2 CH2OOCCH3), or methylcellulose, or methyial (C3H8O2 or CH2(OCH3)2);
n. boiling water;
o. cold water;
p. any combination of the above, in part or whole;
q. any binder, for the purpose of injection molding, con-taining any of the above, in part or whole.

14. The method of claim 1 wherein the step of infiltrating the metal into the precursor comprises:
placing the metal adjacent to the precursor;
heating the metal and precursor to a temperature which promotes wetting of the precursor by the metal.

15. The method of claim 14 wherein the heating step is performed in a vacuum.

16. The method of claim 14 wherein the heating step is performed in an inert atmosphere.

17. The method of claim 14 further comprising further heating to a temperature which promotes microstructural enhancement of the composite.

18. The method of claim 1 further comprising forming a graded microstructure.

19. The method of claim 7 further comprising forming a graded microstructure.

20. The method of claim 1 further comprising consolidating the starting constituents into a precursor having a substantially disconnected microstructure.

21. A composite formed by the method of claim 1.

22. A composite formed by the method of claim 13.

23. A composite formed by the method of claim 18.

24. A composite formed by the method of claim 20.